A reader asks: "Can someone comment on the legality of using my brother's old Snow Leopard DVD to install OS X? My brother has Lion, so why can't he choose to give it to me? It doesn't violate Apple's 1 license per 1 computer policy."

To use Mac OS X or any software, including open source, you implicitly agree to a contract with that software's author. That's the End User License Agreement, or EULA.

This is not correct for GPL-licensed software.

The author of GPL-licensed sofwtare has said, in effect, that they are granting everybody unconditional permission to use (as in to install, optionally to modify, and to run) their software.

There is no agreement required. Everybody has been granted these permissions whether they "agree" or not. There is no contract involved, as this is purely a one-sided grant of permission. It is not as if anyone is going to say ... oh no, I don't have that permission. No siree, not me!

GPL-licensed software authors also grant conditional permission to copy and redistribute their software, but that is another matter. This also doesn't require agreement, as these additional permissions are granted as long as the conditions are met. If the conditions are not met, there is no grant of permission.

"LICENSE: A right granted which gives one permission to do something which he could not legally do absent such permission; 'leave to do a thing which the LICENSOR [the party granting the license] could prevent.'"

A contract, on the other hand, is defined like this:

"a promise, or set of promises, for breach of which the law gives a remedy, or the performance of which the law in some way recognizes as a duty. I Williston, Contracts Section 1. The essentials of a valid contract are 'parties competent to contract, a proper subject-matter, consideration, mutuality of agreement, and mutuality of obligation.' 286 N.W. 844, 846: 'a transaction involving two or more individuals whereby each becomes obligated to the other, with reciprocal rights to demand performance of what is promised by each respectively.' 282 P. 2d 1084, 1088. 'The total legal obligation which results from the parties' agreement as affected by law.' U.C.C. Section 1-201."